Postnasal drip syndrome: This is often the most common cause of a lingering cough, especially when other symptoms like a runny nose or sinus pressure are present. The cough can persist even after the initial infection has resolved due to ongoing irritation of the nasal passages and subsequent dripping of mucus into the throat.

Asthma: Cough can be a predominant symptom of asthma, especially if it worsens at night or with exercise. A negative X-ray does not rule out asthma, as it is a functional rather than a structural condition.
Gastroesophageal reflux disease (GERD): Stomach acid reflux can cause irritation of the throat, leading to a chronic cough. This is more likely if the cough is worse after eating or at night.
Chronic bronchitis: Part of chronic obstructive pulmonary disease (COPD), chronic bronchitis involves inflammation of the bronchial tubes, leading to a persistent cough. A negative X-ray does not exclude this diagnosis, especially in smokers or those exposed to pollutants.
Allergic rhinitis: Similar to postnasal drip syndrome, allergic reactions can cause chronic cough due to the body's response to allergens.

Lung cancer: Although less likely, lung cancer can present with a persistent cough. It's crucial to consider this, especially in smokers or those with a significant smoking history, even with a negative X-ray, as early stages may not be visible.
Tuberculosis (TB): TB can cause a chronic cough and may not always show up on an initial X-ray, especially in its early stages or in individuals with HIV/AIDS.
Pneumonia (atypical or partially treated): Some forms of pneumonia, especially those caused by atypical bacteria, may not show up clearly on an X-ray. Partially treated pneumonia could also lead to a lingering cough.

Cystic fibrosis: More commonly diagnosed in childhood, but can be diagnosed later in life, especially if symptoms are mild. It leads to persistent lung infections and cough.
Sarcoidosis: An autoimmune disease that can affect the lungs, causing cough among other symptoms. It might not be visible on a standard X-ray if the disease is in its early stages.
Eosinophilic pneumonia: A rare condition characterized by an accumulation of eosinophils in the lungs, which can cause a chronic cough.